The invention relates to a method of calibrating a radiation sensor in order to determine the quantity of radiation x(t) that is absorbed by the radiation sensor from the output signal o(t) of the radiation sensor at a given instant t. The invention also relates to a radiation sensor that is arranged so as to carry out a method of the kind set forth, and to an X-ray detector that includes a plurality of such radiation sensors.
Generally speaking, the function of radiation sensors consists in absorbing quanta of, for example, an electromagnetic radiation field so as to convert such quanta into a corresponding output signal such as an electric voltage or charge. When the functional relationship between the output signal and the input signal, that is, the absorbed amount of radiation, is known, the magnitude of the input signal can be deduced from the output signal. Said relationship (conversion characteristic) is typically determined with the aid of a calibration measurement during which known quantities of radiation x(t) are absorbed and the resultant output signals o(t) are measured. A characteristic that constitutes the desired conversion characteristic can be derived from the pairs of values thus obtained.
However, a problem is encountered in that the conversion characteristic of a radiation source generally is not constant but varies as a function of time under the influence of drift phenomena and aging phenomena of the components of the radiation sensor. For example, in the case of a flat dynamic X-ray detector (FDXD) the hardware of the detector induces differences and non-linearities in the neighboring pixels, rows and blocks, which differences and non-linearities become manifest as visible image artifacts. In order to eliminate such differences, calibration with X-rays in many different doses is required as described above. Because of the variation in time of the characteristic of the FDXD detector, such calibrations must be repeated at given time intervals so as to ensure that the desired measuring accuracy is continuously maintained. Such a method for the repeated calibration of an X-ray detector is described, for example in JP-09-018245. The frequent recalibration, however, has the drawback that X-rays that represent a radiation load for staff and equipment must be employed without medical necessity.